From d06e917b9e640bd9b0f20c24f80b03266fe1010b Mon Sep 17 00:00:00 2001
From: Dean Michael Berris <dberris@google.com>
Date: Wed, 4 Oct 2017 05:20:13 +0000
Subject: [XRay][compiler-rt] Use a hand-written circular buffer in BufferQueue

Summary:
This change removes the dependency on using a std::deque<...> for the
storage of the buffers in the buffer queue. We instead implement a
fixed-size circular buffer that's resilient to exhaustion, and preserves
the semantics of the BufferQueue.

We're moving away from using std::deque<...> for two reasons:

  - We want to remove dependencies on the STL for data structures.

  - We want the data structure we use to not require re-allocation in
    the normal course of operation.

The internal implementation of the buffer queue uses heap-allocated
arrays that are initialized once when the BufferQueue is created, and
re-uses slots in the buffer array as buffers are returned in order.

We also change the lock used in the implementation to a spinlock
instead of a blocking mutex. We reason that since the release operations
now take very little time in the critical section, that a spinlock would
be appropriate.

This change is related to D38073.

This change is a re-submit with the following changes:

  - Keeping track of the live buffers with a counter independent of the
    pointers keeping track of the extents of the circular buffer.

  - Additional documentation of what the data members are meant to
    represent.

Reviewers: dblaikie, kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38119

llvm-svn: 314877
---
 compiler-rt/lib/xray/xray_buffer_queue.h | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

(limited to 'compiler-rt/lib/xray/xray_buffer_queue.h')

diff --git a/compiler-rt/lib/xray/xray_buffer_queue.h b/compiler-rt/lib/xray/xray_buffer_queue.h
index bd382a26c64..05e22eece7d 100644
--- a/compiler-rt/lib/xray/xray_buffer_queue.h
+++ b/compiler-rt/lib/xray/xray_buffer_queue.h
@@ -17,8 +17,8 @@
 
 #include "sanitizer_common/sanitizer_atomic.h"
 #include "sanitizer_common/sanitizer_mutex.h"
-#include <deque>
-#include <unordered_set>
+#include <cstdint>
+#include <memory>
 #include <utility>
 
 namespace __xray {
@@ -36,15 +36,30 @@ public:
   };
 
 private:
+  // Size of each individual Buffer.
   size_t BufferSize;
 
   // We use a bool to indicate whether the Buffer has been used in this
   // freelist implementation.
-  std::deque<std::tuple<Buffer, bool>> Buffers;
-  __sanitizer::BlockingMutex Mutex;
-  std::unordered_set<void *> OwnedBuffers;
+  std::unique_ptr<std::tuple<Buffer, bool>[]> Buffers;
+  size_t BufferCount;
+
+  __sanitizer::SpinMutex Mutex;
   __sanitizer::atomic_uint8_t Finalizing;
 
+  // Pointers to buffers managed/owned by the BufferQueue.
+  std::unique_ptr<void *[]> OwnedBuffers;
+
+  // Pointer to the next buffer to be handed out.
+  std::tuple<Buffer, bool> *Next;
+
+  // Pointer to the entry in the array where the next released buffer will be
+  // placed.
+  std::tuple<Buffer, bool> *First;
+
+  // Count of buffers that have been handed out through 'getBuffer'.
+  size_t LiveBuffers;
+
 public:
   enum class ErrorCode : unsigned {
     Ok,
@@ -117,8 +132,9 @@ public:
   /// Buffer is marked 'used' (i.e. has been the result of getBuffer(...) and a
   /// releaseBuffer(...) operation).
   template <class F> void apply(F Fn) {
-    __sanitizer::BlockingMutexLock G(&Mutex);
-    for (const auto &T : Buffers) {
+    __sanitizer::SpinMutexLock G(&Mutex);
+    for (auto I = Buffers.get(), E = Buffers.get() + BufferCount; I != E; ++I) {
+      const auto &T = *I;
       if (std::get<1>(T))
         Fn(std::get<0>(T));
     }
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